Internal combustion engines are subject to emission regulations. In addition to improving in-cylinder designs, using Exhaust Gas Recirculation (EGR), and better controlling combustion, an aftertreatment device is normally needed for reducing pollutants, which include Nitrogen Oxides (NOx), Carbon Monoxide (CO), Hydrocarbon (HC), and Particulate Matter (PM), to required levels. In spark ignitions (SI) engines, fuel and air can be pre-mixed stoichiometrically, therefore, not much PM is seen in exhaust gas, while CO, HC, and NOx are major pollutants. However, in a compression ignition (CI) engine, due to heterogeneous fuel-air mixing and its lean combustion nature, PM and NOx are major components in its pollutants, while CO and HC are relatively insignificant.
In CI engines, in using EGR for adjusting peak combustion temperature, a tradeoff needs to be made between PM level and NOx emission. When both of PM and NOx need to be controlled, normally, two methods are used with an aftertreatment device. One is tuning NOx emission low, and using a high efficiency particulate filter, such as a diesel particulate filter (DPF), for removing PM. The other one is tuning PM level low, and using lean NOx removing technology, such as urea/ammonia Selective Catalytic Reduction (SCR), Lean NOx Trap (LNT)/NOx Absorber (NAC), and Lean NOx Catalyst (LNC), for controlling NOx emission. In the first method, normally the filter needs to be regenerated periodically. The regeneration can be achieved by heating up the filter to 500° C. to 600° C., and the heating energy is provided by burning fuel in an oxidation catalyst, such as a diesel oxidation catalyst (DOC) or a burner. Fuel penalty for filter regeneration depends on engine operating conditions and NOx emission level. When a low NOx emission level is required, e.g. according to US2010 HD standard, NOx emission is limited to 0.2 g/bhp·hour, fuel penalty could be a limiting factor for using the particulate filter method.
The other method needs to remove NOx from lean exhaust gas. As oxygen, NOx is also an oxidant. Therefore, a selective environment must be created more favorably for reactions reducing NOx, since oxygen concentration is much higher than that of NOx. Among all technologies used in reducing NOx in lean exhaust gas, SCR has the highest conversion efficiency, and thus is used broadly. However, in a selective catalyst, there exists a tradeoff between conversion efficiency and selectivity. A catalyst with high selectivity normally has poor conversion efficiency. As a result, to have high selectivity, a device with a large volume is needed when high conversion efficiency is required.
Though no hydrocarbon fuel is required in SCR technology, a urea solution needs to be mixed with exhaust gas. The urea solution is then evaporated and ammonia is generated through hydrolysis and thermolysis of urea. This ammonia generation process requires extra heat energy, and when the heat energy is provided by burning more fuel in engine, this fuel penalty could be more than 3% of total engine fuel consumption, depending on engine operating conditions and emission limits. Additionally, in reducing NOx, urea is consumed. The overall cost of urea consumption and fuel penalty in generating ammonia is comparable with the fuel penalty cost in a particulate filter system.
Different from that in CI engines, in SI engines, when air-fuel ratio is controlled at a stoichiometric level, NOx could have a higher or comparable concentration as oxygen. As a result, even in an oxidation catalyst without good selectivity, high deNOx efficiency could be obtained. This type of catalyst usually is called three-way catalyst, since in such a catalyst, CO and HC reduce NOx, and all three pollutants can be removed from exhaust.
Compared to a SI engine, a CI engine creates a lean exhaust gas due to its lean combustion nature, which causes difficulties in reducing NOx. When the lean exhaust gas becomes rich, an oxidation catalyst can be used to effectively reduce NOx. It is a goal of the present invention to provide a means for reducing NOx and other pollutants in lean exhaust gas by converting a lean exhaust gas to a rich exhaust gas without significantly sacrificing fuel economy. Furthermore, it is a goal of the present invention to provide an exhaust gas aftertreatment system without using a reducing agent other than hydrocarbon fuel.